Implantable medical devices (IMDs), such as cardiac rhythm management systems, are commonly used to provide treatment or therapy to patients. Cardiac rhythm management systems can include pacemakers. Pacemakers deliver timed sequences of low energy electrical stimuli, called pacing pulses, to the heart, via an intravascular lead wire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pacing pulses. Pacemakers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Such pacemakers may also coordinate atrial and ventricular contractions to improve pumping efficiency.
Cardiac rhythm management systems can also include cardiac resynchronization therapy (CRT) devices for coordinating the spatial nature of heart depolarizations for improving pumping efficiency. For example, a CRT device may deliver appropriately timed pacing pulses to different locations of the same heart chamber to better coordinate the contraction of that heart chamber, or the CRT device may deliver appropriately timed pacing pulses to different heart chambers to improve the manner in which these different heart chambers contract together.
Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators include cardioverters, which synchronize the delivery of such stimuli to sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a “shock.” The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, CRT devices, and defibrillators, cardiac rhythm management systems can also include devices that combine these functions, as well as monitors, drug delivery devices, and other implantable or external systems or devices for diagnosing or treating the heart.
Certain physiological analytes impact many of the problems that implantable medical devices are designed to treat. As one example, potassium ion concentrations can affect a patient's cardiac rhythm. Therefore, medical professionals frequently evaluate physiological potassium ion concentration when diagnosing a cardiac rhythm problem. However, measuring physiological concentrations of analytes, such as potassium, generally requires drawing blood from the patient. Blood draws are commonly done at a medical clinic or hospital and therefore generally require the patient to physically visit a medical facility. As a result, despite their significance, physiological analyte concentrations are frequently measured only sporadically.